Resistance-tuned circuit



April 12, 1932. S CABOT 1,853,604

RESISTANCE TUNED CIRCUIT Filed May 28, 1929 ATTEIRNEL:

Patented Apr. 12, 1932 SEWALL (14301, or BROOKLILTE,MASSACHUSETTS RESISTANCE-TUNED CIRCUIT Application filed May 28, 1929. Serial No. 366,689.

One object of my invention is to provide a system .of electrically tuned circuits comprising fixed inductances, resistances and capacities and in addition a variable resistance which determines the frequency to which the system is maximally responsive.

.Heretofore, insofar as I am aware, such systems have been tuned by the variation of inductance, capacity or both, which determines the frequency to which the system is maximally responsive.

In my circuit, however, this tuning is accomplished by the variation of a resistance. In order to accomplish this efiect one of the resistances must be negative. It is well known to the art that a negative resistance may be obtained from the electrodes of an ordinary vacuum tube with appropriate direct current voltages applied.

In my system, as is usual in tuned systems which contain negative resistance in some form, the circuit may be made stable and any sudden shock set up in it will die away in the form of a damped oscillation,-"th'e duration of the damped oscillation being dependent on the value of the negative resistance chosen. With a slightly changed value of negative resistance the circuit becomes a generator of sustained oscillation of the frequency to which it is tuned. Both of these conditions may be of value in radio reception and they last mentioned of. value in radio transmission or the generation of sustained oscillations.

specification, a circuit which may be utilized to tune a radio antenna to be maximally responsive to any desired frequency within a bandof desired frequencies or maybe used to'cause theantenna to send out an-electromagnetic wave of any desired frequency within a band, said frequency being dependent on the particular adjustment of an adjustable resistance. 7

In the figure, the source of negative resistance comprises: a vacuum tube'l, 2, 3 together m having contacts 4 and 10 to determine th connected a fixed inductance L shunted by a sistance for all frequencies provided that is the antenna capacity (which may be aug- In order to more fully disclose my invention, I show in the figure of the drawing which accompanies and forms a part of thiswith sourcesof electromotive force A and B,

direct current polarizing voltages ontheelectrodes 2, 3 and a filament rheostat R to govern the electron emission from the filament. V This structure forms the equivalent of a negative resistance R shunted by a capacity 0 This negative resistance may be given any value within limits by appropriate adjustment of the filament resistance R and the contacts 4 and 10. If the interelectrode capacity C is insufficient for the band of frequencies to be covered it may be augmented by connecting additional capacity between electrodes 2 and3.

In series with electrode 3 to ground 9 are fixed resistance R an adjustable resistance R and the circuit between points'fi and 9 WhlCh may-be made a fixed and constant re- In the circuit between points 6 and 9, C

merited if desired to cover a particular desired band of frequency response) and C is a by-pass condenser of negligible reactance .tothe band of, frequency response. L -is a fixed inductance.

In order to make the circuit maximally responsive to alternating currents of a definite predetermined frequency, the constants thereof are so chosen that for said frequency the effective negative resistance of the circuit 6, 5, 3, 9 is approximately equal to the eifective positive resistance between the points 6,

9, which as aforesaid, is equal to I The electromagnetic constants of the par-( resistance R and the contacts 4 and 10, made 7 "equal to r where A is anumber very small compared to unity.

The resistance between the points '5 and 3 is so chosen and designed as to be equal to The total impedance of the circuit, designed by giving the electromagnetic constants thereof the values above set forth, may be determined for any frequency f as follows If the impedance of that portion of the l \F i Z 0 J GU a K4- mam) (Ka i 3 To simplify the equations the factor is introduced into all expressions for the actual resistances of the portions of the circuits between the pairs of numbered points 39, 35, and 59, indicated on the diagram, and this factor is multiplied by the constants K K or K as the case may be, to get the actual resistances in ohms between each pair of said points.

The equations may be further simplified by writing as m,

R the resistance between the points 3 and 9, as

R the resistance between the points 3 and 5, as

The total impedance of the circuit may then be expressed as circuit betWeen thepoints-8' and 9 (i. e., between the points 3 and 2) is represented by Z the impedance between the points3 and 5 by Zg, and the impedance betweenthe points circuit-will be 5 and 9 by Z5, the total impedance of the If K is unity and K is (1+A) where A is a number very small compared to unity 3;

as aforesaid 1 +Kfm =Zf m (approximately) and the total impedance becomes (approximately) (3) In order to determine the resonant frequency with a known value of K or to determine the value of K for a known frequency, the resistive component of the impedance is equated to zero, viz,

the resistive Component disappears and the I reactive component becomes which is a relatively small quantity dependent on the value A and ensures a relatively large current value with small impressed "electromotive force in the circuit.

The maximum frequency f to which the circuit may be tuned is as follows:

Let

then 7 i Theminimum frequency will be zero.

In order to determine thetransient effects 7 and stability of the circuit, it will be neces- E "at I I= e S w t+w /L 0 cos w t),

where a .A. 2 /L 0 approximately and It will be noted from this that the effects of a shock die out with time if A is a positive number and that the circuit becomes a generator of local oscillations if A is a negative number.

Havingthus described an illustrative embodiment of my invention, without however limiting myself thereto, what I claim is 1. A tunable circuit comprising in combination two circuits connected in parallel, one of said circuits including a capacity and a positive resistance connected in series and a approximately.

the other including an inductance and a posi tive resistance connected in series, means whereby an electromotive force may be impressed in one of said circuits, said positive resistances each being approximately equal,-

in ohms to the square root of the ratio of said inductance to said capacity, an effective negative resistance connected in parallel with said circuits, and means for adjusting the value of the effective negative resistance so that at a definite predetermined frequency said negative resistance is approximately equal in value to said positive resistances.

2. An electrical network comprising a parallel-branch circuit, one branch of which in--;

cludes a serially-connected inductance and resistance and the other a serially-connected capacity and resistance, each of said resistances being approximately equal in ohms to the square root of the ratio of said inductance to said capacity, and a negative resistance connected across the terminals of said parallei-branch circuit of value approximately equal to that of either of said positive resistances.

3. The method of rendering an electric circuit maximally responsive or self-oscillatory at a predetermined definite frequency which consists in adjusting-a positive resistance in said circuit so that the value K in the expression I representing the resistance of a portion ofsaid circuit, is equal to where m=w /L O' w=27r times the frequency of an alternating voltage impressed upon said circuit, C is the capacity between the grid and plate of a thermionic tube included in said circuit, and L is the value of an inductance connecting the plate of said tube'to said circuit.

4:. An electrical network comprising a parallel-branch circuit, one branch of which includes a serially-connected inductance and resistance and the other a serially-connected capacity and resistance, each of said resistances being approximately equal in ohms to the square root of the ratio of said inductance to said capacity, means for impressing an alternating electromotive force on one branch of said parallel-branch clrcuit, a circuit serially connected with said parallel-branch clrcuit, and means included in the last-mentioned circuit for neutralizing the effective positive resistance of said parallel-branch circuit at a definite predetermined frequency.

.5. An electric circuit maximally respon sive or self-oscillatory at a definite predetermined frequency comprising a serially-connected inductance,capacity and variable resistance, a negative resistance connected in shunt to said capacity, the value in ohms of said negative resistance being approximately equal to the square root of the ratio of said inductance to said capacity, and a positive resistance connected in shunt to said inductance, said positive resistance having approximately the same value as said negative resistance.

6. An electric circuit maximally responsive or self-oscillatory at a definite predetermined frequency comprising a serially-connected inductance, capacity and variable resistance, and resistances of opposite sign connected, respectively, in shunt to said inductance and capacity, each of said resistances being approximately equal to the square root of the ratio of said inductance to said capacity.

7. A resistance-tuned circuit comprising an effective negative resistance and a positive resistance connected in series, and means for rendering said effective negative resistance substantially equal in value to the positive resistance of said circuit for a definite predetermined frequency only.

8. A resistance-tuned circuit comprising a fixed inductance, a resistance, and a negative resistance serially connected, a capacity in shunt to said negative resistance, a resistance in shunt to said fixed inductance, and an adjustable positive resistance in series with said negative resistance for rendering the effective value of said negative resistance substantially equal to that of the total positive resistance of the circuit for a definite predetermined frequency only.

9. An electrical network comprising a parallel-branch circuit, one branch of which includes a serially connected inductance and resistance, and the other a serially connected capacity and resistance, each of said resistances being approximately equal in ohms to the square root of the ratio of said inductance to said capacity, a negative resistance and an inductance connected in series with each other and across the terminals of said parallel-branch circuit, a capacity shunt ing said negative resistance, a resistance shunting said inductance, the value in ohms of said last-mentioned resistance being equal to the square root of the ratio of the lastmentioned inductance to said capacity, and the value in ohms of said negative resistance cuit comprising a serially-connected resistance and capacity in one branch and a serially-connected resistance and inductance in the other branch, each of said resistances being equal to the square root of said inductance divided by said capacity, an impedance connected across the terminals of said parallelbranch circuit, said impedance comprising a positive resistance, a negative resistance, an inductance and a capacity, the reactance component of said impedance being very small or negligible for all frequencies within a given band and the resistance components thereof being negative, and adjustable means whereby said resistance component of said impedance is made substantially equal to the positive resistance of said parallel-branch circuit for one only of the frequencies Within said band.

11. A radio system comprising in combination a parallel-branch circuit, one branch of which includes serially-connected inductance and resistance and the other a seriallyconnected capacity and resistance, each of said resistances being approximately equal in ohms to the square root of the ratio of said inductance to said capacity, an antenna electrically associated with said parallel-branch circuit, and a circuit including a source of negative resistance having its terminals conv nected, respectively, to the parallel-branch circuit.

In testimony whereof, I have hereunto subscribed my name this 27th day of May, 1929.

SEWVALL CABOT.

junctures of said being slightly greater than the square root of the ratio of the last-mentioned inductance to said capacity, and an adjustable resistance connected in series with said negative resistance and inductance for rendering the efiective value of said negative resistance substantially equal to that of the total positive resistance of the circuit for a definite predetermined frequency only.

10. A tuned or self-oscillatory circuit comprising in combination, a parallel-branch cir- 

